If my opponent plays Star Crossed and I react with Sacrifice to kill my only unit, does my opponent still need to return a friendly unit?

To resolve this interaction, we must look at how Star-Crossed handles multiple targets when one becomes unavailable.

The Ruling

Yes, your opponent’s remaining legal target (the enemy unit) is still returned to their hand.

When Star-Crossed was played, it created a targeting relationship with both a friendly unit and an enemy unit. By reacting with Sacrifice and moving your friendly unit to the trash, you made that target unavailable. Per Rule 359.3.e.8, if an instruction has more than one target and fewer than all of those targets become invalid or unavailable, the instruction will still execute, operating on the remaining valid targets.

Sequence of Events

1. Opponent plays Star-Crossed: They target your friendly unit and one of their units. Star-Crossed is now pending on the chain.
2. You play Sacrifice: You move your friendly unit to the trash.
3. Chain State: The chain contains [Sacrifice] (top) and [Star-Crossed] (bottom).
4. Resolution:
   • Sacrifice resolves: You draw 2 cards and channel 1 rune.
   • Star-Crossed resolves: It checks for its targets. Because your friendly unit is now in the trash, it is no longer available. However, because the enemy unit remains a legal target, the spell proceeds to resolve, returning that enemy unit to its owner's hand.

Summary: Your opponent’s Star-Crossed still resolves, returning their enemy unit to their hand, while the portion of the instruction targeting your (now unavailable) unit fails to execute.

Note: I could not fully verify this ruling against the retrieved rules.